Adblock is extremely useful; I've depended on it for several years. But at least in recent versions (evident in March or April 2006 and continuing to the present (July 2006, version 0.5.3.043)), it has caused an extreme memory leak in Firefox.

For example with Adblock disabled, running Firefox with about 20 tabs open there is NO memory increase overnight (idle), and Firefox can run for days without quitting and restarting. In contrast, when Adblock is enabled, there is an overnight increase in memory usage of about 180 MB, and after 24 hours or less of normal use, Firefox with Adblock enabled requires so much memory that my whole system slows almost to a stop unless I quit and restart Firefox. (1 GB RAM, dual P3 2 GHz, Win2K).

I don't know if this is a new leak bug in Adblock or is an old bug which is being unmasked now that Firefox's worst memory leaks have been fixed over the past year.

I have verified that Adblock causes severe leaks. Test page:
http://elek2002.vtk.be/
On a fresh profile the page takes a reasonable fixed amount of RAM. With Adblock installed the page gradually uses more and more RAM just being open (the page has some ugly DHTML running a scrolling ticker on the bottom that's probably interacting badly w/ Adblock, but that's no excuse). Closing the tab doesn't free the RAM.